PRESSURE AND GASES
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You are surrounded by air. You are surrounded by pressure. You have heard it many times: "Air has pressure." What is this all about?
Air is made up of molecules. They are so light they float around you without sinking to the ground. There are basically 2 groups of molecules in the air, nitrogen and oxygen. The nitrogen takes up a huge portion of the air: 78%. So 3/4 of the molecules causing the atmospheric pressure are tiny nitrogen molecules. Oxygen makes up 21% of the air. Combined, nitrogen and oxygen make up 99% of what is going into your nose, throat, and lungs at sea level. All the other gases make up only 1% of the atmosphere and most of that is argon.
The exact content of dry air (in percent): Nitrogen 78.084; Oxygen 20.947; Argon 0.934; Carbon dioxide 0.0350; Neon 0.001818; Helium 0.000524; Methane 0.00017; Krypton 0.000114; Hydrogen 0.000053; Nitrous oxide N2O 0.000031; Xenon 0.0000087; Ozone Trace to 0.0008; Carbon monoxide Trace to 0.000025; Sulfur dioxide Trace to 0.00001; Nitrogen dioxide NO2 Trace to 0.000002; Ammonia Trace to 0.0000003.
Google Amadeo Avagadro to see what he contributed to gas behavior.
Not only are these molecules floating around you, but they are vibrating. The average speed of a molecule at sea level and at a temperature of about 60 degrees F is over 1000 miles per hour. They vibrate because they are warm. If you heat them up they will move faster. If you cool them down they will slow down. If you could cool them down enough they would stop vibrating entirely. The temperature where the molecular motion comes to a halt is so low (-273 degrees Celsius, -459 degrees F.) scientists have never been able to get to that point. They have come close. Using laser beams to slow down sodium atoms scientists have come to within a billionth of a degree of absolute zero.
If you took a vibrating jackhammer and put it against your body it would make you less appealing. Even though we cannot see molecules of nitrogen and oxygen, billions are vibrating against our bodies and they are also capable of doing great damage. They are tiny and there are so many of them they are capable of killing you. An oxygen molecule at room temperature next to your skin the effect resembles a bullet fired from a gun. Even though a bullet is bigger and would penetrate doing significant damage, the gas molecules number in the billions on each square inch and in combination have the capability of doing far more destruction. That is where the air pressure comes from: Tiny molecules of nitrogen and oxygen vibrating against every square inch of your body.
Draw a square inch on a piece of paper. The billions of molecules vibrating against it exert a force of 14.7 pounds at sea level. On a common sheet of paper (8" x 11") there are 88 square inches. The air pushing against that paper has a force of 1,294 pounds! Yet, when you hold the paper in your hand it doesn't seem to have a pressure of over a half a ton on it. Because there are molecules on the other side of the paper pushing with the same force the pressure on both sides is equal. If you were to take the oxygen and nitrogen molecules away from one side of the paper there would be an obvious and violent reaction. It's no wonder airplane windows have to be strong so they don't get blown out because the molecules on the outside get less as the plane gains altitude.
The average atmospheric pressure at sea level is 14.7 pounds per square inch due to the approximately 60 miles of air above it. (Above 15 miles there is so little air the weight is insignificant.) That may be abbreviated as "14.7 psi." To make things easier that number can be referred to as 1 atmosphere or 1 atm. As one gets closer to space the number of molecules gets less. The total number of molecules in the square inch column is about 400 sextillion (400 followed by 21 zeros). At an altitude of 18,000 feet there are about 1/2 of the molecules found at sea level. The pressure at 18,000' is 1/2 of what it is a sea level. That would make it 7.4 psi or 1/2 atm. At that altitude there are so few oxygen molecules you could not survive. Commercial airplanes commonly fly above 30,000 feet and stepping outside one would not be conducive to one's well being.
In AOPA Pilot (2/2006) meteorologist Brian Guyer described his launching of weather balloons twice per day in Virginia. The balloons carry GPS radiosondes that transmit the temperature, humidity, pressure, wind speed, wind direction, and GPS location as it ascends in the atmosphere. When Brian fills the balloon with helium it has a diameter of 6 feet. About 1 1/2 hours after launch it reaches and altitude of 100,000' and has a diameter of 100'! That's when the balloon exploded. The radiosonde is parachuted to the ground. If you find one send it back to NOAA.
Something else divers need to know about gases: They move from one place to another. They usually flow from where there is a lot of a gas to where there is little. For example, if you had gasoline fumes (molecules) in a bottle in your kitchen and you opened the bottle the gasoline molecules would immediately move from the bottle into the kitchen air and make it stink. Because the pressure of the gasoline in the bottle was high, and the pressure of the gasoline in the kitchen prior to opening the bottle was zero, the gasoline molecules flowed from high pressure to the low pressure. Gases always do that. They flow from high pressure to low pressure and they will continue to flow out of the bottle until the pressure becomes the same in and out. (At that point the flow would continue but the molecules would go in and come out of the bottle at the same rate.) The same would be true with an automobile tire. The pressure in the tire is high compared to the pressure outside the tire. It might be 40 psi. If you forced a nail through the rubber wall of tire the gas would flow from the high pressure interior to the low pressure outside until the tire was flat and the pressure was equal. Incidentally, Graham's Law states that gases flow from areas of higher pressure to areas of lower pressure.
Thomas Graham (1805-1869)
Oxygen molecules are slightly heavier than nitrogen molecules. In fact, 32 nitrogen molecules weigh as much as 28 oxygen molecules. Since oxygen is heavier one would think the oxygen would be found at the bottom of your living room with the nitrogen floating on top of it. But remember, gases flow from high pressure to low. If the bottom of your living room were pure oxygen it would represent high pressure compared to the ceiling where no oxygen would be found. Since gases flow from high pressure to low (or zero), the oxygen molecules would vibrate themselves from the floor to the ceiling until the pressure was equal. The nitrogen on the top would do the same. So if you test the air in your living room, the mixture will be the same from floor to ceiling: 78% nitrogen and 21% oxygen.
The same thing happens to a gas that sits on top of a liquid. Let's say you have a glass of water in your hand. The oxygen and nitrogen molecules in the air above the water will actually flow into the water until the pressure is the same both in the water and in the air. The molecules will dissolve in the water just as if they were sugar. That's Henry's Law. William Henry was the man. If a fish were in the water and consumed a little of the oxygen dissolved in the water to stay alive the pressure would drop. More oxygen would flow into the water to keep the pressure in the air the same as that in the water. If the fish exhaled a little carbon dioxide, a waste product gas, there would be an increase in the pressure of that gas in the water. Since the pressure of the carbon dioxide in the air is very low, the gas from the fish would race to the surface and enter the room air. That's the way the fish keeps getting oxygen from the air, and the glass of water does not become poisoned by the fish's exhaled carbon dioxide.
Gases also compress easily. In class, you took a syringe and tried to compress the pool water. It was almost impossible. When you did the same for air some of you were able to compress it by over 1/2 with just one hand! That is an important point for divers. If we were made out of liquids and solids we would have little problem with pressure under water. Our legs, being only composed of fluids and solids, do not compress during descent. However, we do have several gas spaces that compress as we go down in the water. They will cause discomfort and damage unless measures are taken to keep the pressure the same inside and outside those spaces. In the next few lessons a large amount of time will be spent on the ways of protecting the areas in your body that contain gas such as your ears, sinuses, lungs, stomach, intestines, and external items added to the diver such as the face mask.
How much of a pressure change would you expect underwater? Remember it was said you would have to ascend in the atmosphere 18,000 feet to reduce the pressure by 1/2? You only have to go up from 33 feet in salt water to the surface in order to create the same affect! (Because fresh water is lighter, it takes 34'.) Pressure underwater changes rapidly, very rapidly. You can drive down a mountain and "fix" your ears without being in any great hurry. Diving down in the ocean does not afford you the same time luxury. You must make adjustments rapidly and repeatedly to enjoy diving and avoid injury! It is important to master this in the pool before diving in open-water.
Not that it has much to do with diving, meteorologists do not use 14.7 psi or atmospheres to measure air pressure. Instead they use millibars and/or inches of mercury (Hg). The average pressure of the atmosphere is 1013.2 mb or 29.92" of mercury at 15 degrees C (59 degrees F) at sea level. If there is an airmass coming that contains more pressure, that is, more molecules banging on each square inch, the weather will tend to be drier, sunnier, and cooler. The pressure may be 1028 millibars. If a warmer, wetter, and stormier air mass arrives it will have less molecules per square inch and the pressure may drop to 988 millibars. The highest recorded pressure was 1084 mb or 32.01" Hg. That was on a cold winter day in Siberia when the molecules were so close together there were many more of them banging on each square inch. The lowest recorded pressure was in Typhoon "Tip" in Japan. The pressure dropped to 870 mb. or 25.69" Hg. The lowest recorded pressure in the US was from hurricane Wilma in 2005. The pressure dropped to 882 mb.
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